When we have sex, our bodies release certain neurochemicals that cause us to bond with the person we have sex with.
For instance, our bodies release oxytocin. Oxytocin is a neurochemical that does a number of things in humans, but it should be noted here that it is addictive. That is, oxytocin is as much of an addictive substance as is alcohol or nicotine.
Most people readily describe the emotional effects of oxytocin as having “a warm and fuzzy feeling towards someone”. If you gave someone a shot of pure oxytocin, they would experience a rush of warm and fuzzy feelings, among other things.
So what does all this mean? It means that when you have sex with a person, your body releases an addictive chemical that you come to associate with that person. If you cease having sex with that person, you will be able to go a few days with no problem. Then the withdrawl symptoms will set in and you will yearn for him or her (you are really yearning for more oxytocin, but your mind doesn’t know that).
This pattern is why so many couples break up, are happy with their break up for a few days, and then plummet into yearnings for each other. Not realizing that they are chemically addicted to each other, they think their yearnings mean they are in love with each other. So, they get back together again. Only to face the same problems that caused them to break up in the first place.
The moral of the story, if there is one, is this: Be careful who you sleep with. If you sleep with them often enough, whether inside marriage or outside of marriage, you will become addicted to them. That is especially true for women: Estrogen multiplies the bonding effect of oxytocin.
I am not making an argument here for restricting sex to marriage, but rather am merely saying that sex has consequences we don’t always think about, but should. Sex, after all, is something that evolved in us not just for procreation, but (at least in humans) also for bonding us to each other.